This invention relates to an adaptive defrost control system for use in a temperature controlled device, such as a refrigerator. The adaptive defrost control system utilizes various types of sensed information to control the energization of a defrost heater for de-icing the coils of an evaporator.
It has been recognized that energy consumption and adverse temperature fluctuations within a refrigerated space can be minimized by de-energizing a defrost heater as soon as a frost load has been removed from an evaporator. It is generally desirable to defrost as infrequently as possible, but it is not desirable to allow very large frost loads to develop because they require more time and electrical energy to remove, thus reducing the operating efficiency of the cooling device.
Optimum defrost operation thus requires that a balance be struck between the competing considerations of system operation with a frosted evaporator, the energy consumed in removing a frost load from the evaporator, and the acceptable level of temperature fluctuation caused by a defrosting operation.
In some prior refrigerator defrosting systems, a predetermined number of counts must be accumulated before a defrosting operation is initiated. These counts may be defined as either a cycling of the compressor or as an opening of the refrigerator door while the compressor is operating. However, such a control is not responsive to the duration of door openings, and the number of counts needed to initiate defrost is fixed.
Other types of prior defrost controls have integrated the run time of the compressor and the duration of door openings and initiated the defrost cycle after a fixed cumulative amount has been reached. Alternatively, an electromechanical control has been used to integrate the freezer temperature and the ratio of the compressor on/off time to initiate the defrost cycle. These types of systems rely upon expensive electrical or mechanical components, the first utilizing a coulometer and the second utilizing a combination of thermostatic switches and a thermal time delay relay to perform the integrating function. Moreover, these types of systems do not utilize the previous defrost history, which is an important factor in providing an efficient method of defrosting the evaporator.
Another type of defrost control system combines a relative humidity sensor with either the number of occurrences or the total time duration of cabinet door openings or compressor operation. In each case the combined effect of the refrigerator conditions alters the time interval between defrost cycles. However, this type of defrost system does not utilize both the number and duration of total door openings and the total compressor run time which accumulates between defrost operations.
Other types of defrost systems control the interval between defrosting operations based upon the time required for the defrost heater to raise the evaporator to a predetermined temperature during the previous defrosting operation. The net result of such a system is that the defrost interval will be inversely related to the heater "on" time during the previous defrost operation.
Still another type of defrost control provides a minimum amount of time between defrost operations. This control utilizes a conventional time based defrost timer which is interrupted prior to defrost to allow a demand defrost sensor to take over. The defrost operation is prevented until the sensor indicates that a predetermined frost load has been accumulated.
These and other types of defrost controls suffer from the disadvantages of not taking into account the number and duration of door openings and the previous defrost history.